1. Field of the Invention
The present invention relates to staggercasting methods and apparatus.
2. Background of the Invention
Current digital television transmission standards in the United States, as proposed by the Advanced Television Systems Committee (ATSC) dated Sep. 16, 1995, incorporated by reference herein, use a single carrier modulation technique: eight level vestigial sideband modulation (8-VSB). Because it is a single carrier modulation technique, it is susceptible to signal degradation in the communications channel, such as fading caused by multipath and other signal attenuation. While some such fading may be compensated by channel equalization techniques, if the fade is long enough and severe enough, then the receiver will lose the signal and the demodulator system will lose synchronization. Reacquiring the signal, and resynchronizing the demodulator can take several seconds and is quite objectionable to a viewer.
To overcome this problem, a first ATSC proposal permits creation of a second communications channel by permitting use of a more robust modulation technique for a limited period of time, e.g. less than 10%. For example, a 2 or 4-VSB modulation technique may be used for selected frames. A second ATSC proposal permits a more robust encoding technique, e.g. trellis encoding, while maintaining an 8-VSB modulation technique. Such a system permits improved performance with compatible receivers while maintaining backwards compatibility with existing receivers.
Another known technique for overcoming fading is staggercasting. PCT Application No. US02/22723 filed Jul. 17, 2002, by K. Ramaswamy, et al., and PCT Application No. US02/23032 filed Jul. 19, 2002 by J. A. Cooper, et al., incorporated by reference herein, disclose staggercasting communications systems. Staggercasting communications systems transmit a composite signal including two component content representative signals: one of which is delayed with respect to the other. Put another way, one of the component content representative signals is advanced with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, the advanced-in-time component content representative signal is delayed through a delay buffer so that it becomes resynchronized in time with the other component content representative signal. Under normal conditions, the undelayed received component content representative signal is used to reproduce the content. If, however, a signal fade occurs, then the previously received and advanced-in-time content representative signal in the delay buffer is used to reproduce the content until either the fade ends and the composite signal is available again, or the delay buffer empties. If the delay period, and the associated delay buffer, is large enough then most probable fades may be compensated for.
PCT Application No. US02/22723 filed Jul. 17, 2002, by K. Ramaswamy, et al., and PCT Application No. US02/23032 filed Jul. 19, 2002 by J. A. Cooper, et al. also disclose a staggercasting system in which one of the component signals in the composite signal represents the content at a higher quality than the other component signal. In this arrangement, the lower quality component signal is advanced in time relative to the higher quality component signal. As described above, at the receiver under normal conditions, the undelayed received component, which in this case is the higher quality component signal, is used to reproduce the content. If, however, a signal fade occurs, then the previously received and advanced-in-time content representative signal, which in this case is the lower quality component signal, in the delay buffer is used to reproduce the content until either the fade ends and the composite signal is available again, or the delay buffer empties. This permits reproduction of a higher quality signal under normal conditions, and reproduction of a lower quality signal in the presence of a fade event. Because the lower quality signal requires fewer bits to transmit, the overhead required to provide fade resistance is decreased.